Paper No. 83-1
Presentation Time: 9:00 AM-5:30 PM
THE LOWERMOST CAMBRIAN AND SSFS OF THE XIAOLANTIAN SECTION IN THE CHENGJIANG AREA, YUNNAN, SOUTH CHINA
KONO, Sena1, ISOZAKI, Yukio2, SATO, Tomohiko3, ZHANG, Xingliang4 and LIU, Wei4, (1)Department of General Systems Studies, University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo, 153-8902, Japan, (2)Department of General System Studies, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo, Tokyo, 153-8902, Japan, (3)Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan, (4)Northwest Univ, Taibailu 229, Xian, 710069, China, kono@ea.c.u-tokyo.ac.jp
The earliest Cambrian stratigraphy of small shelly fossil (SSF) has been best analyzed in Yunnan, South China, where various depositional settings were recognized; i.e. from shallow-water (e.g. Meishucun section) to relatively deep-water (e.g. Laolin, Xiaotan section) facies. This study examined litho and bio-stratigraphy of the lowermost Cambrian at the Xiaolantian section near Maotianshan in the Chengjiang area, which was deposited in an intermediate depth setting. The lowermost Cambrian of the Xiaolantian section is ca. 300 m thick, and is composed of the Zhongyicun Member (phosphorite), Dahai Mb (dolomite), and Shiyantou Fm (mudstone), in ascending order. The Zhongyicun Mb (ca. 30 m) is lithologically further subdivided into Units 1-5 in ascending order. Unit 1 (ca. 11.5 m thick): interbedded massive phosphatic dolomite and thin phosphorite; Unit 2 (ca. 1.1 m): dolomitic phosphorite with thin black mudstones (ca. 15 cm); Unit 3 (ca. 3.4 m): phosphorite (ca. 10-20 cm) interbedded with thin dolomite; Unit 4 (ca. 5.6 m): phosphorite and thin bedded dolomitic phosphorite; Unit 5 (ca. 4.8 m): thin-bedded phosphorite (ca. 2-3 cm).
SSFs were recovered from 10 horizons in the Zhongyicun Mb (from Units 1 and 5) and 1 from the Dahai Mb. Two distinct SSF assemblages were recognized; Assemblage A only from the lowermost part of Unit 1, composed mainly of Acanthocassis orthacanthus, Protohertzina anabarica, and Protohertzina unguliformis. Assemblage B from Unit 5, composed mostly of Eohalobia diandongensis, Paracarinachites sp., and Lomasulcachites macrus. Conotheca spp. and Siphogonuchites spp. commonly occur from Units 1 and 5 and also from Dahai Mb but not from Units 2-4. The Assemblages A and B by and large correspond to previously reported the 1st and 2nd SSF assemblages, respectively; however, it is noteworthy that some representative taxa of the 2nd assemblage, such as Siphogonuchites spp. and Lopochites spp. with relatively complicated structures, occur also in Assemblage A from the lowest Zhonyicun Mb. Some “advanced-form” SSFs may have diversified much earlier than previous believed.
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